1. Field of the Invention
The present invention relates to a method for compensating a failing nozzle in a printhead comprising a series of print elements for operation in an inkjet printing process in which a colorant is applied to a receiving medium for locally changing an optical density, thereby printing an image. The invention further relates to an inkjet printing system comprising control means configured to apply the invented method.
2. Description of the Related Art
Inkjet printing systems are getting increasingly sophisticated. Additional features relating to speed and print quality are continuously introduced for enhancing the range of applicability of inkjet printing systems. Furthermore, the printheads, that form the heart of the inkjet print process, are perpetually improved. Still, it occurs that a print element in a printhead does not discharge an ink drop according to predetermined specifications. Either no ink is applied on positions where an ink dot is supposed to be applied, or an ink dot is applied on a different position from where it is supposed to be applied. The cause of this malfunctioning is often found in the clogging of a nozzle, comprised in the print element, from which the ink is discharged, in residual ink on a nozzle plate of the printhead, or in the introduction of air in the ink channel. Whatever the cause, a non- or malfunctioning print element is known as a failing nozzle. There exist techniques that remediate a failing nozzle, depending on the cause of failing, but these are not the subject of the present invention.
Obviously, a failing nozzle implicates an inferior print quality, since an ink dot can not be provided as required by a control unit of a printer. This ink dot is referred to as a missing dot. The print quality consequence may be debilitated in various ways, depending on the way a printhead is applied in the inkjet printing system. In some systems, a printhead is reciprocated in a scanning direction to print swaths, each swath contributing to a printed image on a receiving sheet-like material. This material is stepwise transported, relative to the beam along which the printhead reciprocates, in a subscanning or transport direction, that is substantially perpendicular to the scanning direction. Alternatively, the printhead beam is moved stepwise across a receiving substrate. In either system, the array of print elements extends in the subscanning direction and a print mode, or print strategy, may be devised wherein a print position on the receiving material is served more than once, each time by a different print element. These print modes are known as multipass print modes. The print data for a specific print position that is served by a failing nozzle of one print element may then be transferred to another print element that is also serving that specific print position. Such a substitution method is the subject of U.S. Pat. No. 5,124,720. Of course, also single pass print modes are known. For these, no similar substitution method is available.
In other print systems, a configuration of one or more printheads, each comprising an array of print elements, extends in a direction substantially perpendicular to a transport direction, which is the direction in which the receiving substrate and the printhead are movable relative to each other. This is also known as a line-type ink jet configuration. The configuration is made as wide as the receiving material on which an image is printed, hence the name page wide printhead array, and the position of the printheads is fixed in the direction perpendicular to the transport direction. Each print position on the substrate is served by a single print element only and the print strategy is essentially a one-pass strategy. Substitution methods as described above, are not applicable for these systems. A method for diminishing the effects of failing nozzles is provided in U.S. Pat. No. 5,587,730. In this patented invention, a second printhead is placed behind a first printhead for each applied colorant, thereby providing a spare nozzle for each print position. However, in most cases, this is not a very economical solution.
In order to compensate a failing nozzle in any of the systems mentioned above, different methods exist that provide additional ink in the neighbourhood of a missing dot, i.e. a dot that would and should be printed by the print element associated with the failing nozzle, if it would function normally. In European patent 1060896 B, a method is described to provide an addressable correction point in the vicinity of a missing dot. This correction point receives the image forming material from a different print element than the print element with the malfunctioning nozzle. In this way, the optical density that the printed material is supposed to achieve, is not affected by the failing nozzle. Another method to retain an optical density is the provision of marking material of another color on the same print position as a missing dot, as elucidated in U.S. Pat. No. 5,581,284. This compensates at least some of the lightness deviation that is caused by a missing dot, although other color properties, such as chroma and hue, still deviate. In all these methods, print data associated with the print element having a failing nozzle is transferred to another print element, applying marking material either or not on the same position as the missing dot. If a print element is capable of applying more than one dot size, a transfer of print data may imply a change of dot size at a neighbouring print position.
However, despite all these possibilities for compensating a failing nozzle, linear imperfections in inkjet printed images still occur. These are especially apparent if the images are printed in a one-pass print system. In spite of an applied compensation, an optical density in a uniform area shows lines of lower optical density, i.e. light lines, but also lines of higher optical density, i.e. dark lines on positions in the printed image associated with failing nozzles. These lines are also referred to as undercompensated failing nozzles and overcompensated failing nozzles, respectively. The present invention addresses this non-uniformity associated with failing nozzles, which is considered to be a problem for some applications of inkjet printing. An object of the present invention is to reduce this non-uniformity.